Configuration Examples for ATM-to-Ethernet Interworking Functions

The following sections describe sample configuration cases to illustrate how the interworking functions are performed when a 1:1 and an N:1 association exists between DSLAMs and the PPPoE access concentrator (B-RAS application):

Single DSLAM Connected to a PPPoE Access Concentrator Example

Consider a topology in which a DSLAM multiplexes multiple PPPoE over ATM sessions over the Ethernet aggregate network using interworking functionality between PPPoE over ATM and PPPoE. Figure 39 shows a sample configuration scenario in which one DSLAM is connected to one PPPoE access concentrator. The subscriber access nodes are connected using ATM links to the DSLAMs. In this circuit, a 1:1 relationship exists between the DSLAM and the PPPoE access concentrator (PPPoE major interface). Because of the presence of a 1:1 association between the DSLAM and the access concentrator, segregation of VLAN IDs is not required. A one-to-one mapping is configured between the user port and VLAN. The access node is assigned a unique VLAN identification to a port using either a unique S-VLAN ID or a unique (S-VLAN ID, C-VLAN ID) pair. The uniqueness of the mapping is maintained in the access node and across the aggregation network. The B-RAS PPPoE access concentrator demultiplexes the PPPoE sessions.

Figure 39: Single DSLAM Connected to a PPPoE Access Concentrator

In such a case, the PPPoE access concentrator uses the maximum number of subinterfaces configured on a PPPoE interface using the pppoe sessions command to limit the maximum number of IWF PPPoE sessions. The B-RAS tracks the IWF flag in a PPPoE PADR packet it receives from the access concentrator, which the PPPoE client forwarded to the access concentrator during the discovery stage. The presence of the IWF tag causes the PPPoE access concentrator to accept duplicate PPPoE sessions with the same MAC address of the DSLAM are permitted by default. The suboption for the IWF-Session DSL Forum VSA (26-254) contains the field code of 0xFE in PADR packet and its length field is set to 0x00.

Multiple DSLAMs Connected to a PPPoE Access Concentrator Example

Consider another scenario in which the access node assigns the same S-VLAN ID to a group of ports. Figure 40 shows a sample configuration scenario in which multiple DSLAMs are connected to a single PPPoE access concentrator. This configuration pattern is denoted as N:1 VLAN to indicate many-to-one mapping between ports and VLAN. Sample criteria for grouping multiple ports with the same S-VLAN ID comprise the same originating virtual path, same service, or same destination service provider. In this topology, multiple DSLAMs, which aggregate sessions from multiple PPPoE clients, are connected to one access concentrator.

Figure 40: Multiple DSLAMs Connected to a PPPoE Access Concentrator

Although in such a scenario, in which a N:1 DSLAM and PPPoE access concentrator relationship is exhibited, normally a VLAN grouping is performed to segregate the traffic from each DSLAM. Such a grouping of VLAN IDs is a regular network administration practice. If such a grouping of VLAN IDs is made, a 1:1 relationship between DSLAM and the PPPoE access concentrator is created that enables each access concentrator to serve a VLAN group.